OK, here goes, and I won't even charge you for the advice ;)
If you're really that unhappy where you are then I'd agree that it's a good idea to move. It's mature to recognise that mismatch. In a poor cultural fit then you're not performing and you're not serving either employer or client.
One way to spin the opportunity to your advantage would be to set up as a sole trader, or Private Limited concern, and have the firm employ you either on a consulting or interim management basis.
Benefits - You can demonstrate an entreprenurial spirit by setting up on your own, it also shows an awareness of the risks of being independent, the returns are potentially better if you have a decent accountant. You're broadening your skillset and demonstrating mobility in client industry, ie you're not IT specific. That makes you more marketable for a potential employer.
In the medium term you can try for follow on work, either with the same firm or another client which demonstrated some sales competence. (although how valuable that is might be questionable, but it demonstrates willingness). You might find that you enjoy the independent approach.
Problems with it - You might be seen as a ultural fit risk if you can't fit in with your current firm, so less employable. It might be seen as a lack of commitment. You might be seen asa lacking depth, the counterpoint to the broadening that I mentioned as an advantage.
I wouldn't recommend going in as a permie if you want to break back into consulting later.
Business Link can give you some advice on setting up on your own. It is a risk if you're not comfortable about neworking and selling, but in all honesty I'd prefer seeing someone prepared to give it a go.